I have been thinking about upgrading my tanks to either 3, 4 or 5 gallons (maybe even a 10 gallon with dividers?) and adding a filtration system. HOWEVER, I had multiple tanks a long time ago and I seem to recall having a lot of trouble until the tank was established. I remember "the cycle" and I have vague memories that this was a scary and dangerous thing. Some questions...

Which size tank is big enough for the fish, yet small enough to easily clean? Is it really all that big a difference for a betta to be in a 4 gallon tank as opposed to 2.5 gallon?

Can I still do 100% water changes while using a filter or do I need to get this cycle going?

Is establishing a tank as difficult as I remember? I remember quite a few dead fish in the beginning and people telling me that was normal. I don't want to lose my bettas!

Is establishing a tank worth the trouble? Or is it just as hard to siphon the gravel and do a 50% water change once a week when you have a filter?

I'll try to answer this to the best of my ability, although I'm hoping some others may chime in.

I personally feel that establishing a filtered tank is great for your fish and not to mention makes things somewhat easier for you.

The one problem with filtering betta tanks is water flow. A lot of times these filters produce a very high current in which case you will have to make a filter baffle(basically a sponge or a water bottle put over the filter mouth as to decrease the output of the water).

No, doing a 100% water change wouldn't be a good idea. It will remove the established bacteria.

Sometimes bigger is better and can be much easier to care for once the nitrogen cycle has established....its a lot easier than it sounds...as long as you provide the good bacteria with what it needs the nitrogen cycle will happen.....the nitrifying bacteria are kinda like fish.....they need dechlorinated oxygenated water(filter), a place to live (surface area) and food(livestock and plant byproducts)

This bacteria is air born-one convert the ammonia to nitrite and another converts nitrites to nitrate-they are sticky and adhere to everything inside the tank, in the top layer of substrate and in the filter media-very little are in the water column-so water only changes will not stall or slow the cycle...over cleaning can....

You can safely cycle with the fish provided that you are willing to make the needed water changes to keep them safe

Establishing the nitrogen cycle is a lot easier if you have water testing products on hand-a good Freshwater master test kit to test for: ammonia, nitrite, nitrate, pH is needed especially if you do the Fishless cycle method-with the Fish-in cycle you don't have to have a test kit but it will help and help you know when the cycle is completed (4-8 weeks) however, if you have a pet shop close by they usually will test your water for free.

IMO/E
In small filtered tanks-1-3gal the nitrogen cycle only changes the percent needed not the number of needed water changes to maintain water quality

In 5-10gal filtered tanks the nitrogen cycle can decrease the number of needed water changes to maintain water quality

You make water change based on ammonia, nitrite level to keep them 0.25ppm or less..0ppm is ideal and with the nitrate you want to keep this 5-20ppm-too high of nitrate can also be bad for the fish.

I go by a 50% rule with water changes-you make water changes to keep ammonia, nitrite, nitrate under control but you also have other things in the tank that we don't test for that can also be bad for our livestock that has to be removed in a closed system as well and this is the- DOC's or dissolved organic compounds.

IMO/E-with this species of fish due to their long heavy fins-sometimes the filter in small tanks can cause fin problems and you need a filter to establish the nitrogen cycle
Sometimes this species will not tolerate or do well in too big of a tank because of their heavy fins-especially when they get older-the weight of the fins cause them to tire with too much space and water movement and some Betta will thrive and bloom with the space and water movement...each one is different......

I'm going to be doing an unfiltered, 10 gallon divided. Mostly because my boys need heaters asap, and I only want to get one, and also because I LIKE doing water changes, and I have a lot of time on my hands. So it's different for everyone. Also, one of my boys came with injured pectoral fins, and I think even a small current might be too strong for him. I am treating him with Aqua salt right now in a smaller container, though.